The present invention relates to screen cylinders, particularly of the type intended for use in screening cellulosic materials, for example, in the paper and pulp industry, and particularly relates to a perforated screen cylinder formed of a ceramic or other formed materials, such as graphite or plastic, and methods of constructing the screen cylinder.
Cylindrical perforated screens have long been employed in the paper and pulp industry for cleaning the pulp of impurities, for example, fiber bundles, sticks and other coarse particles. When using cylindrical screens to accomplish this, a pressure difference is maintained between the feed or inlet on one side of the screen and an accept region on the opposite side of the screen such that the accepts are passed through apertures in the screen for collection and the rejects are maintained on the inlet side of the screen for flow to a reject discharge.
Screens of this type have been made of high wear-resistant steel material, occasionally using an investment casting process. Obviously, this has led to great expense in the manufacture of screen cylinders. Particularly, such process does not lend itself to ready formation of screen cylinders of different diameters and lengths inasmuch as entirely new molds must be provided for each change in dimension or configuration. For these and other reasons, it has been found desirable, according to the present invention, to use a ceramic or other formed type of material in the formation of the cylindrical screen. In certain applications, a ceramic material is the material of choice, principally because of its high resistance to wear, lubricity and other characteristic properties. It has also been found desirable to provide screens of this type in a number of different sizes, i.e., diameters, and lengths for different screening apparatus and applications, and also in different configurations, all from a plurality of a single basic cylindrical screen section or element.
According to the present invention, identical screen cylinder sections or elements are formed of a ceramic material or other formable materials. This application describes a preferred embodiment of screen cylinder, namely a ceramic screen cylinder, but it will be appreciated that the invention has applicability to other types of materials for forming the cylinders, e.g., such as graphite, plastic and the like. Various numbers of the elements may be joined one to the other to form screen cylinders of different diameters and lengths, and hence different configurations. That is, a single basic element may be formed of predetermined size and configuration and joined with other identical elements to form screen cylinders of different sizes. Forming a plurality of identical elements, particularly of a ceramic material, is much less expensive in comparison with the expense necessary to manufacture steel screen cylinders of different diameters and lengths as in the prior art. To accomplish the foregoing, the present invention provides a plurality of identical, generally arcuate, elements, for example, each formed of a ceramic material. Each element has a plurality of screen openings or apertures therethrough and is joined to adjacent elements along opposite longitudinally extending edges to form a cylinder. While preferably each ceramic section or element has either or both of its interior or exterior surface in the configuration of an arc of a circle, it will be appreciated that one or both of the interior and exterior surfaces may be formed coincident with chords of a circle or may both be formed of arcs of concentric circles of different diameters.
In accordance with one aspect of the present invention, the elements are provided with interconnecting means along their longitudinally extending edges which enable these sections to pivot slightly relative to one another about an axis passing through the interconnecting means. This enables a plurality of the elements to be joined one with the other along their longitudinal edges to accommodate each element's use in screen cylinders of different diameters. That is, cylindrical screens of different sizes may be formed by employing a greater or lesser number of the identical elements and joining them one to the other along the longitudinally extending edges, the pivotal connection along the edges accommodating the increase or decrease in diameter of the particular screen cylinder being formed in comparison with a screen cylinder of a median diameter for screen cylinders of that type.
Preferably, the interconnecting means along the longitudinal edges are in the form of tongue and grooves. In this manner, the elements may be joined one to the other by sliding one element relative to another element with its tongue or groove engaging the complementary groove or tongue, respectively, of the adjacent element, there being sufficient play when the tongue-and-groove connection is effected such that the one element is pivotal relative to the other element about an axis generally parallel to the axis of the screen cylinder. When the elements are connected one to the other to form the cylinder, connecting rings are provided at opposite ends to maintain the elements connected one to the other and to maintain the screen in its cylindrical shape.
To provide screen cylinders of different lengths, each of the elements may be cast of a predetermined length and two or more generally cylindrical sets, each comprised of a plurality of such elements, may be disposed endwise relative to one another. A connector ring is disposed therebetween connecting the two sets of cylindrical elements in end-to-end relation.
In screen cylinders used in the pulp and paper industry, the walls of the cylinder are alternately flexed inwardly and outwardly by rotor induced pressure forces. Therefore, to accommodate this cyclical force, cinch rods are provided in the present invention about the sectional cylinder at circumferentially spaced positions and connected to the end connecting rings to place the elements under axial compression such that the cyclical forces will not exceed the compressive forces induced by the tensioned cinch rods.
Further, to prevent collapse or other damage to the cylinder in the event one or more of the elements is damaged or collapses, a support tube is provided, preferably about each cinch rod. Under normal operating conditions, an end of each tube is slightly axially spaced from the associated connecting ring, leaving a slight clearance space. Upon collapse of an element, the tensioned cinch rod draws the end connecting rings toward one another with the result that the tube ends butt the connecting rings, preventing further axial movement of the connecting rings toward one another.
In a preferred embodiment of the present invention, there is disclosed an apparatus for forming a screen cylinder comprising a plurality of generally arcuate elements formed of a ceramic material and having spaced opposite edges. Means are provided defining screen openings in each element and additional means interconnect the elements one with the other along their edges to form a substantially continuous screen cylinder.
In a further preferred embodiment in accordance with the present invention, there is disclosed apparatus for forming a screen cylinder having an axis comprising a plurality of generally arcuate elements formed of a ceramic material, each having spaced opposite edges extending generally in the direction of said axis. Means are provided which define screen openings through each element and additional means interconnect the elements one with the other along their edges to form a substantially continuous screen cylinder. The inner or outer surface of each element has a predetermined radius of curvature, the elements, in one assembly of a predetermined number of the elements, forming a generally continuous screen cylinder of one diameter, and, in another assembly of a predetermined number of elements and with one or more additional or one or more fewer elements forming a generally continuous screen cylinder of a larger or smaller diameter, respectively.
In a still further preferred embodiment of the present invention, there is disclosed a screen cylinder having an axis comprising a plurality of generally arcuate elements each having spaced opposite edges extending generally in the direction of said axis. Means are provided defining screen openings through each element and additional means interconnect the elements one with the other along their edges to form a substantially continuous screen cylinder, the inner or outer surface of each element having a predetermined curvature about an axis parallel to and not coincident with the axis of said cylinder.
In a still further preferred embodiment of the present invention, there is provided a method for constructing a screen cylinder having an axis comprising the steps of casting from a ceramic material a plurality of arcuate elements, each having substantially opposite edges extending generally in the direction of the axis and screen openings therethrough, and joining the elements together along said edges to form a substantially continuous screen cylinder.
Accordingly, it is a primary object of the present invention to provide a novel and improved screen cylinder for use in the pulp and paper industry and methods of forming the screen cylinder wherein the cylinder is formed of a ceramic or other formable material and may be readily and easily constructed of identical elements in different screen diameters, lengths and configurations.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.